1. Field of the invention
The invention is in the field of installing a cable in a tubular cable conduit. More particularly, the invention comprises an improved method for installing a cable in a cable conduit with the aid of the drag forces of a flowing medium and a pulling plug which can be used in such a method.
2. Discussion of Background
References [3], [4] and [5] disclose first type installation methods with which a cable can be installed in a conduit with the aid of drag forces exerted on the cable by a medium, such as compressed air, flowing concurrently and rapidly. In this case, where the cable is fed into the pressurised space from the ambient pressure, a pressure difference occurs. According to this known technique, this pressure difference is compensated for near the place where it occurs by feeding the cable into the pressurised space with mechanical (references [3] and [5]) or hydrodynamic (reference [4]) means. From reference [5] it is, in addition, furthermore known to subject cables having a certain stiffness to an additional pushing force near the conduit entrance in order to compensate for a deficiency of drag forces in an initial part of the conduit, as a result of which larger installation lengths can be achieved in one fell swoop. The advantage of these known methods of the first type is that the drag forces exerted on the cable and having the effect of tensile forces are in general sufficiently equally distributed over the entire length of the cable. Nevertheless it has been found that installation is sometimes fairly suddenly hampered by upsetting or buckling. This appears to occur, inter alia, in the case of somewhat stiff cables which may have so-called intrinsic curvatures, that is to say they have the tendency to move in a curved manner, especially at the position of the foremost cable end. This results in an increased friction between cable and inside wall of the conduit and increases the risk of upsetting during the installation process. In a second type of known methods for the installation of cables in cable conduits, a tensile force is generally exerted on the foremost end of the cable. This can be achieved, for example, with a winching wire introduced beforehand into the conduit and attached to the cable end, as known from reference [1] (see under C.), or with the aid of a pulling plug attached to the cable end, which plug provides, in the conduit, a suction seal to the conduit and is energised with compressed air supplied from the feed-in end of the conduit, as known from reference [2]. Installation methods of this second type have the advantage that, by their nature, they do not suffer from the buckling effect, while, in addition, the foremost cable end is always guided in the right direction. In addition, a pulling plug energised by compressed air has the advantage of an implicit pressure drop compensation at the conduit feed-in end. It is true that these ways of installation have the disadvantage that, as a consequence of the cable tension present, the tensile force required may start to increase rapidly in the bends and undulations virtually always present in the cable conduit. This forms, in general, the limitation of the length of a cable to be installed in one piece in this way. A combination of the principles of both types of installation methods which could eliminate the disadvantages of each type separately and could possibly result themselves in an improved method with which still greater installation lengths could be achieved in one fell swoop is not readily possible. In such a combination, a winching device at the conduit removal end requires additional protective measures against the stream of air flowing out at high velocity and, in addition, a good coordination with the cable feed equipment at the conduit feed-in end. In addition it is necessary, as an additional step, for a winching wire to be introduced into the conduit beforehand from the conduit feed-in end. The known pulling plugs energised with compressed air, which have to seal the duct as well as possible for a satisfactory operation, render a flow of air of any significance along the cable impossible in principle.